Screen for a free and a restricted viewing mode

ABSTRACT

A screen with a transmissive image generator with pixels which is operable in at least two operating modes, B1 for a free viewing mode and B2 for a restricted viewing mode, and with an illuminating apparatus comprising a backlight that has a planar extension and radiates light in a restricted angular range, a plate-shaped light guide arranged in front of the backlight and provided with outcoupling elements on one of the large surfaces and/or within its volume, the light guide being transparent to at least 70% of the light emitted by the backlight; and light sources arranged laterally at edges of the light guide. In mode B2, the backlight is switched on and the light sources are switched off, and in mode B1 at least the light sources are switched on. In interaction with the transmissive image generator the illuminating apparatus constitutes an advantageous screen permitting the operating modes.

PRIORITY CLAIM

This application is a continuation-in-part of U.S. patent applicationSer. No. 16/476,846, filed Jul. 9, 2019, which is a National Phase entryof PCT Application No. PCT/EP2018/067473, filed Jun. 28, 2018, whichclaims priority from German Patent Application 10 2017 006 285.4, filedJun. 30, 2017, the disclosures of which are hereby incorporated byreference herein in their entireties.

FIELD OF THE INVENTION

In recent years, great strides have been made in enlarging the visualangle of LCDs. Frequently, however, there are situations in which such avery large angular viewing range of a screen can be a disadvantage.Increasingly, information such as banking data or other privateparticulars and sensitive data is becoming available on mobile devicessuch as notebooks and tablet PCs. Accordingly, users require somecontrol of who is allowed to see such sensitive data; they must be ableto choose between a wide viewing angle in order to share displayedinformation with others, e.g., when looking at vacation snaps or readingadvertisements, and, on the other hand, a narrow viewing angle needed tokeep displayed information private.

A similar problem is encountered in vehicle manufacturing: Here, whenthe motor is running, the driver must not be distracted by imagecontents such as digital entertainment shows, whereas a front-seatpassenger would like to watch them during the ride. This requires ascreen that can be switched between corresponding display modes.

DESCRIPTION OF PRIOR ART

Accessory films based on micro-louvers have already been used on mobiledisplay screens to provide optical data protection in what is known as aprivacy mode. However, such films are not capable of being switchedbetween modes but have to be applied and removed manually. Also, theyhave to be carried separately from the display screen when not in use.Another substantial disadvantage is the light loss accompanying the useof such louver films

U.S. Pat. No. 6,765,550 describes such privacy protection provided bymicro-louvers. Here, the biggest disadvantages are the need tomechanically attach or remove the filter, and the light loss in theprotected mode.

U.S. Pat. No. 5,993,940 describes the use of a film the surface of whichis provided with small, regularly arranged prismatic strips to create aprivacy mode. The development and fabrication of this film are quitecomplicated.

In WO 2012/033583, switching between free and restricted viewing iseffected by the triggering of liquid crystals arranged between so-called“chromonic” layers. This involves a light loss, and implementation israther complicated, too.

US 2009/0067156 discloses a great number of ideas to configure anilluminating system and a display device. In particular, the versionillustrated there in FIGS. 3A and 3B uses two backlights consisting ofwedge-shaped light guides, and an LCD panel, where the posteriorbacklight 40 is intended to positively create a wide illuminating angle,and the anterior backlight 38 is intended to positively create a narrowilluminating angle. It remains unclear, however, in what manner thebacklight 38 is meant to create a narrow illuminating angle withoutconverting the light having a wide illuminating angle, originating frombacklight 40, essentially into light having a narrow illuminating anglewhen it passes backlight 38.

Regarding the configuration shown in FIG. 5 of US 2009/0067156, itshould be noted that the two light guides 46 and 48 each produce “narrowlight”, i.e. light with a narrow illuminating angle. Conversion of thelight in light guide 48 into “wide light”, i.e. light with a wideilluminating angle, is only achieved by means of a partial mirror 50,which has to be provided with prism structures in a complex process.This conversion extremely diminishes the light intensity, because thelight that at first exits in a narrow illuminating angle (the only lightavailable) is then spread out into a wide illuminating angle, as a ruleinto the semispace. As a result, the brightness will be reduced by afactor of 5 or higher (as regards luminance), depending on theparameters. Thus, this configuration is of little practical relevance.

In the embodiment according to FIG. 7 of US 2009/0067156, a phosphoruslayer that converts UV light into visible light is an absolute must.This is rather laborious to do; and given the aim to get sufficientlight from the backlight to illuminate an LCD so that it can be read,one needs very high UV intensities. Therefore, this configuration isexpensive and complicated; shielding off the UV radiation alone makes itimpracticable.

US 2012/0235891 describes a highly complex backlight in a screen.According to FIGS. 1 and 15, this design employs not only several lightguides but also other complex optical elements such as microlenselements 40 and prism structures 50, which convert the light coming fromthe posterior illumination on the way to the anterior illumination. Thisis expensive and complicated to implement, and it involves a light loss.According to the version shown in FIG. 17 in US 2012/0235891, both lightsources 4R and 18 produce light having a narrow illuminating angle, withthe light radiated by the posterior light source 18 first beinglaboriously converted into light with a large illuminating angle. Thiscomplex conversion greatly diminishes brightness, as noted alreadyabove.

According to JP 2007-155783, special optical surfaces 19 that aredifficult to compute and to manufacture are used to deflect light intovaried narrow or broad regions depending on the light incidence angle.These structures are similar to Fresnel lenses. Furthermore, there existinactive edges, which deflect light into unwanted directions. Thus, itremains uncertain whether really useful light distributions can beachieved.

For achieving restricted vision as taught by GB 2428128 A, additionallight sources, which are arranged at a conspicuous distance from thescreen and illuminate a hologram attached to the screen, are used tooverlay the lateral view with special wavelengths. The disadvantageshere are the necessary spacing of the light sources from the screen, andthe complexity of making suitable holograms.

US 2013/0308185 describes a special light guide provided with steps,which radiates light on a large area into various directions, dependingon the direction in which it is illuminated from an edge. In interactionwith a transmissive imager, e.g., an LC display, a screen that isswitchable between a free and a restricted viewing mode can be produced.Here, a disadvantage, among others, is that the restricted view effectcan only be created either for a left/right or a top/bottom direction,but not for left/right and top/bottom directions simultaneously asneeded for certain payment actions. In addition, some residual light isstill visible from blocked viewing angles even in the restricted viewingmode.

In WO 2015/121398, the applicant discloses a screen of the kinddescribed at the start that, for switching between modes, essentiallyhas scattering particles provided in the volume of the respective lightguide. The scattering particles chosen there, consisting of somepolymerizate, as a rule suffer the disadvantage, however, that light iscoupled out from both of the large surfaces, whereby about half of theuseful light is radiated in the wrong direction, i.e. toward thebacklight, from where, due to the design, it cannot be recycled to asufficient degree. Moreover, the scattering polymerizate particlesdistributed in the volume of the light guide may possibly, especially athigher concentrations, lead to scattering effects that diminish theprivacy protection effect in the protected mode.

The methods and arrangements mentioned above have, as a rule, the commondisadvantages that they distinctly reduce the brightness of the basicscreen, and/or require an active, but at least a special, opticalelement for switching between the modes, and/or are complicated andexpensive to fabricate, and/or degrade resolution in the free viewingmode.

Light guides in backlight designs for LCD screens are typically madefrom plastic materials (e.g., PMMA or polycarbonate) by methods suitablefor cost-effective mass production such as injection molding. The lightguides made in this way have, as a rule, coarse macroscopic surfaceblemishes well discernible with the naked eye, which couple out thelight. Due to such coarse textures, the haze levels of such light guidesare rather high, as a rule, possibly adopting values of 15% or evenmarkedly higher. Scattering properties of that kind make it impossiblefor light already focused on the large surfaces to penetrate such alight guide without being appreciably influenced. In addition,state-of-the-art light guides invariably require additional opticallayers to radiate evenly distributed light of adequate usefulness forLCD panels. Means used for this purpose, are, e.g., diffuser films,brightness enhancement films (BEF, DBEF), or reflectors that return thelight coupled out of the back surface. Especially due to the coarseoutcoupling structures, often designed as scattering elements, the useof such light guides without a diffuser is not possible without a markedloss in light homogeneity.

SUMMARY OF THE INVENTION

Departing therefrom, a problem of the invention is to describe anilluminating apparatus by which, in interaction with a screen, securepresentation of information can be implemented by way of an optionallyrestricted viewing angle, with a second operating mode enabling freevision with a viewing angle that is as unrestricted as possible. Theinvention is intended to be implementable by simple means and aslow-priced as possible. In both operating modes, the highest possibleresolution is to be visible, with particular preference of the nativeresolution of the screen used. Further, in an embodiment, the inventivesolution is to cause the least possible light loss.

This task is solved by means of a screen with an illuminating apparatus,the screen to be used in at least two operating modes, viz B1 for a freeviewing mode and B2 for a restricted viewing mode, comprising atransmissive image generator with pixels or subpixels and furthercomprising a backlight of planar extension that radiates light in arestricted angular range. An eligible restricted angular range maygenerally be any range that is smaller than the semispace in front ofthe backlight; preferably intended here, however, is an angular rangee.g., of +/−20 or 30 degrees horizontally and/or vertically or as a coneabout the normal to the surface or a selectable direction vector on thebacklight, with small light levels of less than 1% to 5% maximumbrightness being negligible. Initially, the terms “vertically” and“horizontally” generally relate to two mutually perpendicular specifieddirections on the surface of the backlight or on a large surface of alight guide which, in operation, correspond to a direction that isactually horizontal or vertical relative to the position of a viewerand, thus, of the earth's surface, depending on the orientation of thescreen used with the illuminating apparatus, the said screen beingfixed, as a rule.

The illuminating apparatus further comprises a plate-shaped light guidethat is located in front of the backlight as seen in the viewingdirection, has at least four narrow sides (edges) and two largesurfaces, and is provided with outcoupling elements on at least one ofthe large surfaces and/or within its volume, the said light guide beingtransparent to at least 70% of the light emitted by the backlight, andwith light sources arranged laterally at the edges of the light guide,preferably at one or both of the long edges of the light guide.

The number of outcoupling elements per surface area and their extensionare selected such that the light guide, on at least 50% but preferably80% of its surface, has an average haze value of less than 7%, butpreferably less than 2% or, with special preference, less than 1%,measured according to ASTM D1003 (the measurement here being based onthe more common procedure A with a haze meter as reference), whereby thelight radiated by the backlight at least in B2 mode in a restrictedangular range is only negligibly, if at all, scattered at angles outsidethe said angular range when it passes the light guide. “Negligibly”means, e.g., that, due to the low haze, within a horizontal angle of,say, 40° from the surface normal, the light density includes a share ofmaximally 1% contributed by scattering, which is radiated by theilluminating apparatus at an angle of 0°.

In the fabrication of the light guide, the outcoupling elements can, inprinciple, be distributed in or on the light guide in various ways tomeet adaptable and pre-set conditions. The outcoupling elements arelocally limited structural changes in the volume and/or on the surfacesof the light guide. Therefore, the term “outcoupling element” expresslyexcludes any additional optical layers applied on the surfaces of thelight guide, such as diffusion, reflection, polarization recycling or(dual) brightness enhancement layers ((D)BEF). These additional layersnot considered as outcoupling elements are joined to the light guide atthe edges only, whereas they merely loosely rest on the large surfaceswithout forming a solid physical whole with the light guide. Bycontrast, any lacquers or varnishes applied on the large surfaces andjoining to them by chemical reactions form an inseparable, solidphysical whole with the light guide; therefore, such lacquers orvarnishes are not additional layers within the sense described above.

The structure of the outcoupling elements can be given so that theeffect of each outcoupling element is at least approximately known, andproperties of the light guide or of the light exiting it can bespecified in a targeted manner by a predetermined distribution of theoutcoupling elements.

According to the invention, a distribution of the outcoupling elementson at least one of the large surfaces and/or within the volume of thelight guide is specified in such a way that light irradiated by thelight sources into the light guide and coupled out of the light guide bythe outcoupling elements satisfies the following conditions:

-   -   at least 50% of the quantity of light outcoupled at one of the        large surfaces in an angular range between −50° and +50°        relative to a peak luminance direction of said large surface is        radiated in an angular range between −20° and +20° with respect        to the peak luminance direction of said large surface and        relative to one or two specified directions that are        perpendicular to each other and to the peak luminance direction,        and/or at least 70% of the quantity of light outcoupled at the        large surface in an angular range between −50° and +50° relative        to the peak luminance direction of said large surface is        radiated in an angular range between −30° and +30° with respect        to the peak luminance direction of said large surface and        relative to one or two specified directions that are        perpendicular to each other and to the peak luminance direction,    -   at least 50% of the quantity of light coupled out of the light        guide is coupled out either in the direction leading away from        the backlight or towards the backlight, and    -   in a projection direction parallel to the surface normal of the        light guide, each pixel covers at least two of the outcoupling        elements at least partially, and    -   in the projection direction a fill factor of the outcoupling        elements is 50% of the area of a large surface of the light        guide or less.

The peak luminance direction of a large surface of the light guide isdefined by the direction into which the maximum amount of light, e.g.measurable as luminance, is directed at the center of the area of thelarge surface, i.e., at the centroid. In case there are more than onesuch directions, the peak luminance direction is defined as thedirection closest to the surface normal direction of the large surface.

The fill factor of the outcoupling elements is defined as the surfaceamount of the projected area of the large surface of the light guide,which is covered by outcoupling elements, seen along the projectiondirection. In other words, according to the invention, not more than 50%of said projected area is filled with the area of outcoupling elements.Typically, the fill factor would range between 10% and 15%.

The one or two specified directions are on the one hand perpendicular tothe peak luminance direction and on the other hand to each other, ifthere are two specified directions. They are preferably defined asfollows: A first specified direction lies within a plane that comprisesa vector defining the peak luminance direction. In a best mode, saidplane may further be characterized by intersecting the large surface ofthe light guide in an intersecting line that is parallel to one of thelong edges of the light guide, provided the latter has a rectangularshape. In other embodiments, however, the intersecting lines may not beparallel to any of the long edges of the light guide. Once the firstspecified direction is defined and fixed, the second specified directionis defined by default, since it is perpendicular to the first specifieddirection as well as to the peak luminance direction.

When light is outcoupled at a large surface of the light guide thismeans that the light exits the light guide, defined by its spatialdimensions, at that surface, although the microscopic outcouplingprocess might already take place within the volume of the light guide,e.g. when the outcoupling elements are located within the volume of thelight guide.

The two operating modes B1 and B2 further differ in that, in mode B1,the backlight is switched on and the light sources (on the edges of thelight guide) are switched off, whereas in mode B1 at least the lightsources (on the edges of the light guide) are switched on. Taken intoaccount here is only the light originally radiated by the light sourcesinto the light guide and subsequently radiated by the light guide viathe outcoupling elements, with radiation taking place almost exclusivelyvia the outcoupling elements.

The specified directions may correspond to the above-mentioned verticalor horizontal directions in an external reference frame.

In mode B1, as described above, at least the light sources on the edgesof the light guide are switched on, whereas the backlight may beswitched on or off.

The desired properties of the outcoupling elements essential for theinvention with regard to their number per unit area, their shape andextension in three dimensions, and their distribution over at least oneof the large surfaces and/or the volume of the light guide can bedetermined, e.g., by means of an optics simulation software such as“LightTools™” available from Synopsis™ or other suppliers, and then bephysically implemented accordingly.

In the prior art, the radiation characteristics described above are notachieved without the use of additional layers such as reflectors, BEF,DBEF, prism films or diffusers. Here, the special radiationcharacteristics serve, in particular, to achieve a sufficientlyefficient light gain, because a screen equipped with the inventedilluminating apparatus is typically looked at within a narrow verticalangular range of −20° to +20° or −30° to +30° only. At the same time,one must take care to achieve the low haze levels essential for theinvention, so as not to impair the privacy protection effect in mode B2.

Furthermore, according to the invention, in a projection directionparallel to the surface normal of the light guide in the projectiondirection parallel to the surface normal of the light guide, eachpixel—or, if a pixel is composed of subpixels—subpixel covers at leasttwo of the outcoupling elements at least partially. This helps to reduceor even suppress optical artifacts that may occur due to the opticaloverlay of the structure of the outcoupling elements with the opticalstructure of the transmissive image generator (e.g., an LCD panel), suchas unwanted Moiré effects, color sparkling, etc. Due to the opticalmixing of light passing through the at least two outcoupling elements inthe smallest image unit of the image generator, i.e., the pixels, or ifavailable, the color sub pixels (e.g., R, G, B), such artifacts aredecreased in visibility. Practically, it may be advisable that in saidprojection direction parallel to the surface normal of the light guide,there are parts of or the whole surface of even three, four, five ormore than five outcoupling elements located under each pixel or subpixelof said transmissive image generator, whichever of these is smaller, inorder to support the aforementioned positive effect of such anarrangement even further.

Additionally, the inventive feature that in a projection directionparallel to the surface normal of the light guide, the fill factor ofthe outcoupling elements 50% or less of the area of a large surface ofthe light guide, helps further to reduce optical artifacts and alsosupports the negligible disturbance of the propagation of the focusedlight in private, restricted operation mode B2. A fill factor of 50%means that half of the area in the projected plane is filled withoutcoupling elements.

Outcoupling elements may be provided on both the large surfaces and, asan option, additionally in the volume.

In an embodiment, the light guide comprises or consists of sometransparent thermopolymer, plastic or thermoelastic material or ofglass.

Preferably, the distribution of the outcoupling elements on at least oneof the large surfaces and/or within the volume of the light guide isspecified in such a way that the light coupled out reaches a luminancehomogeneity of 70% on at least 70% of the light guide surface. For thispurpose, the luminance homogeneity can be defined as Lv^(min)/Lv^(max),i.e., the ratio of the lowest to the highest luminance value per unitarea. Another rule for defining luminance homogeneity is given in“Uniformity Measuring Standard for Displays V1.2” by the DeutschesFlachdisplay-Forum.

Furthermore, it is of advantage for some applications if the saidrestricted angular range is configured asymmetrically relative to thesurface normal of the backlight. The asymmetric configuration ispreferably implemented in one of the specified directions. This ishelpful, in particular, in motor vehicle applications, e.g., if a screento be combined with the invented illumination apparatus to form aso-called center information display is arranged on the dashboard abouthalfway between the driver and the front-seat passenger. In this case,the restricted angular viewing range that is exclusively free for thepassenger must be configured asymmetrically, i.e. directed towards thepassenger. Here, the specified direction in which the asymmetry isprovided corresponds to the horizontal.

The outcoupling elements typically have maximum dimensions of 100 μm,preferably between 1 μm and 30 μm.

The outcoupling elements for light outcoupling from at least one of thelarge surfaces of the light guide comprise or preferably consist ofmicrolenses and/or microprisms and/or diffractive structures and/orthree-dimensional structural elements and/or scattering elements withtheir largest dimension having a maximum extension smaller than 35micrometers, but preferably smaller than 15 micrometers. In case ofdiffractive structures, these may be a hologram or a grating/diffractiongrating.

Alternatively, the outcoupling elements themselves may merely have theouter shape of microlenses, microprisms, scattering elements and/ordiffractive structures. In this case, they may be configured especiallyas cavities formed in the volume of the light guide. The cavities may beevacuated, but preferably are filled with some gaseous, liquid or solidmaterial, the material having a refractive index that differs from, andpreferably is lower than, that of the material of the light guide. Byfilling the cavities with material and selecting the material one caninfluence the light conduction and outcoupling. Alternatively oradditionally, also the haze value of the material preferably differsfrom, and preferably is higher than, that of the light guide. Thisconfiguration has the advantage of higher efficiency in lightoutcoupling.

Alternatively, and in a technically easier way, the cavities can beformed if the light guide is made by joining two substrate layers thatare preferably similar. They can be joined chemically, e.g., by adhesivebonding. The cavities, then, are formed as recesses on at least one ofthe boundary surfaces of the substrate layers.

If the outcoupling elements are provided on at least one of the largesurfaces of the light guide, they are favorably fashioned from somepolymer, plastic or glass, with the structure having been embossedthereon by means of a tool. This is possible, e.g., in mass productionby applying a UV-hardening material—e.g., a varnish, a monomer etc.—onto a light guide substrate, the said material being structured bymeans of a tool and hardened, e.g., polymerized, under UV irradiation.Other radiation-hardening materials may be used as well. Forming therecesses to implement the outcoupling elements is possible, e.g., bymechanical, lithographic or printing processes, but also by materialdeposition, material conversion, material abrasion or materialdissolution.

In that way, e.g., grating structures, microprisms—convex (with theplastic parts protruding from the surface) and/or concave (embossed orrecessed within the surface layer of the structured plastic)—, otherthree-dimensional structural elements of other shapes, or evenmicrolenses can be mass-produced at low cost. Both concave and convexstructures can be employed equally.

The light guide or its substrate finally may contain at least 40 wt.-%,or preferably 60 wt.-%, polymethylmethacrylate, related to its weight.Alternatively, the material contained may be polycarbonate, e.g.

The backlight comprises or consists of, e.g., of a planar emitter,preferably another light guide with further light sources arrangedlaterally or on the rear surface, plus at least one light collimatorintegrated in, and/or arranged in front of the planar emitter, such asat least one prism film and/or at least one privacy filter (micro-louverfilter).

Accordingly, the backlight may, in principle, be designed similar to anLED backlight, e.g., as a sidelight, edge light, direct LED backlight,edge LED backlight, OLED or some other planar emitter on which, e.g., atleast one permanent privacy filter (with micro-louvers) is applied.Other versions use devices known as directed backlights.

In another favorable embodiment of the invention, in mode B1, as afunction of specified limiting angles σ, γ, the outcoupled light exitingfrom the light guide at an angle β will, at every point of the lightguide surface in angular ranges satisfying the conditions of 80°>β>γand/or −80°<β<−σ, with 10°<γ<80° and 10°<σ<80°, preferably with γ=σ=40°,measured against a predefined direction passing through a midpoint ofthe surface of the light guide and in at least one of the two specifieddirections, have maximally 80% or, with particular preference, maximally50% of the light intensity of the light exiting from such a point of thelight guide surface along said predefined direction. The predefineddirection may be the surface normal to the light guide. However, it isalso possible to define other useful defined directions, such as thedirection into which the light guide couples the most luminance out. Themidpoint of the surface usually is the centroid of the surface, in arectangle for example the crossing point of the two diagonal linesconnecting corner points. Frequently herein, the specified direction isthe vertical orientation. Without loss of generality, here, a negativeangle is assigned to the side on which the light is coupled in; i.e., anangle of −90° corresponds to a direction from which incoupling takesplace. Herein, the limiting angles σ, γ are definitely specifiedaccording to the optical performance desired for the respectiveapplication. In case of the particularly preferred limiting anglesγ=σ=40°, the light intensity condition then only applies to anglesbetween −40° and −80° as well as between 40° and 80°, as measured versussaid predefined direction. The smaller the limiting angles σ, γ, themore light will, in the one or two specified directions, be concentratedonto the vertical bisector. In a car, for example, where, in mode B1,the driver and the front-seat passenger look at a screen with theinvented illumination apparatus at relatively well-definable viewingangles, one would rather select limiting angles σ, γ smaller than 40°.In a laptop, by contrast, due to the foldability of the screen and theuniversal application scenario regarding the viewing angles of differentpersons, it would make sense to have limiting angles around 40° orgreater. The 80° limit may possibly be shifted to 70°.

In this way, if the invented illumination apparatus with added screen isinstalled in a car, one can achieve, e.g., a reduction of disturbingreflections on the windshield, especially during night-time drives.Furthermore, meeting the above condition results in a conspicuousefficiency of outcoupling from the light guide, and that without the useof any focusing layers such as prism films or the like.

Using the invented illuminating apparatus is particularly advantageousin connection with a screen that can be operated in at least twooperating modes, viz B1 for a free viewing mode and B2 for a restrictedviewing mode. The transmissive image generator is preferably arranged infront of the light guide.

Further, it is of advantage for the invention if one uses an imagegenerator composed of pixels which themselves consist of subpixels, andif each dimension of the said outcoupling elements, i.e., height, depthand width, is smaller than the minimum of width and height of thesubpixels of the image generator employed, i.e., smaller than theminimum of these two quantities. Preferably, each dimension of the saidoutcoupling elements, i.e., height, depth and width, is even smaller bya factor of 1.3, 1.5 or 2.0 than the minimum of width and height of thesubpixels of the image generator used. In this way, the image gets morehomogeneous, and superimpositions of structural patterns and subpixelpatterns can possibly be avoided.

Consequently, for a screen with an image generator is provided withpixels that are assemblies of subpixels, it is advantageous if in in theprojection direction parallel to the surface normal of the light guide,each subpixel covers at least two of the outcoupling elements at leastpartially, preferably fully.

Another practical embodiment of the invented screen includes anotherlight guide (made of glass or plastic, e.g.) that is arranged (seen inthe viewing direction) in front of the image generator and provided withmeans for light outcoupling, and into which light can be fed laterallyby light sources. The outcoupling means used here are, e.g., thosedescribed above, or of a kind known in prior art, such as nanoparticlesof titanium dioxide, barium sulfate etc. in suitable sizes andquantities—as described, e.g., in WO 2015/121398 A1 and WO 2017/089482A1—, which are homogeneously distributed in the volume of the lightguide. With this embodiment, any unintentional residual light in Mode B2in angular ranges that are actually protected from being viewed can beinterfered with or glared to such an extent that no contrast is visibleand no image can be perceived from disallowed angles. Here again, theoutcoupling elements may be fashioned in the form of cavities or oninterfaces, so that, when used in a car, e.g., they make sure thatinformation displayed can only be seen by the front-seat passenger butnot by the driver, in that radiation is restricted to the respectivepartial space.

The light sources eligible are adapted to radiate colored or whitelight, and they can radiate light of a color that is not present in theimage displayed by the transmissive image generator.

Alternatively it is possible for the light sources to radiate light of acolor that is present in the image displayed by the transmissive imagegenerator or is close to such a color in the color spectrum. Finally, itis feasible for the light sources to radiate light of a colorapproximately corresponding to a color complementary to a color presentin the image displayed by the transmissive image generator.

“Colored light” is understood especially to be visible light that is notwhite, e.g., light of red, green, blue, turquoise, cyan, magenta oryellow color. Further, this light can optionally be radiated at variousbrightness levels. Moreover, it is possible for the chromaticproperties, e.g., the color and/or brightness, of the light emitted bythe light sources to be modulated in time. In addition, the lightsources may comprise different light sources, such as, e.g., RGB LEDs inLED rows radiating light of different colors and/or differentbrightnesses either simultaneously or at different times, and/orstaggered.

Arranged on the top side of the image generator and/or on at least oneof the large surfaces of the light guide as well as on at least one ofthe privacy filters, if any, can be means for reflection diminishing orscattering, e.g., an antiglare and/or antireflection coating.

The invented screen may also comprise further optical elements toenhance the efficiency of the illuminating apparatus; such additionalelements may include retardation plates, wire grid polarizers,polarization dependent mirrors, switchable mirrors and/or partialmirrors or the like, e.g., for introducing a polarization recyclingfunctionality and/or to reflect back to the image generator light thatwas radiated by the light guide towards said backlight of theillumination apparatus.

The invented screen can, to particular advantage, be used in a motorvehicle to optionally display image contents only to the front-seatpassenger in mode B2, or simultaneously to the driver and the front-seatpassenger in mode B1. The former is helpful, e.g., if the front-seatpassenger is watching entertainment contents that might detract thedriver's attention.

A screen according to the invention can be used to enter as well as todisplay sensitive data, such as PINs, e-mails, SMS text messages orpasswords, at ATMs, payment terminals or mobile devices.

In all embodiments mentioned above, the said light sources may be LEDsor LED rows or laser diodes. Other versions are feasible and are withinthe scope of the invention.

Furthermore, the desired restricted angular ranges for restrictedviewing in mode B2 may be defined and implemented separately for thehorizontal and vertical directions. In the vertical direction, forexample, a larger angle than in the horizontal direction, or else norestriction at all, might be useful, say, if, at an ATM, persons ofdiffering body heights are to see an image, whereas sideways viewing isto remain greatly or completely restricted. For POS payment terminals,on the other hand, safety regulations frequently necessitate viewingrestrictions in mode B2 both in horizontal and vertical directions.

On principle, the performance of this invention remains unaffected evenif the parameters described above are varied within certain limits.

It is understood that the features mentioned before and those to beexplained below are applicable not only in the combinations stated butalso in other combinations or as stand-alone features without leavingthe scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, the invention will be explained in more detail with reference tothe accompanying drawings, which also show features essential to theinvention, among others, and in which

FIG. 1 is a sketch illustrating the principle of light that is laterallycoupled into a light guide being outcoupled from the lower large surfaceof the light guide, with the outcoupling elements being provided on thesaid lower large surface, and with the light exiting from the upperlarge surface of light guide;

FIG. 2 is a sketch illustrating the principle of light that is laterallycoupled into a light guide being outcoupled from the upper large surfaceof the light guide, with the outcoupling elements being provided on thesaid upper large surface, and with the light exiting from the upperlarge surface of the light guide;

FIG. 3 is a sketch illustrating the principle of light emitted by abacklight passing a light guide;

FIG. 4 is a sketch illustrating the principle of the inventedilluminating apparatus in interaction with an image generator in mode B1for a free viewing mode;

FIG. 5 is a sketch illustrating the principle of the inventedilluminating apparatus in interaction with an image generator in mode B2for a restricted viewing mode;

FIG. 6 is a schematic representation of the definition of the verticaldirection of the angle θ to be measured;

FIG. 7 is a graph of the relative brightness of the light outcoupledfrom the light guide, measured in the vertical direction;

FIG. 8 shows an embodiment of the invented illuminating apparatus ininteraction with an image generator, wherein, in front of the imagegenerator as seen in the viewing direction, another light guide isarranged that is provided with means for outcoupling light and intowhich light can be fed laterally by light sources;

FIG. 9 shows the vertical brightness distribution across an angularrange for a state-of-the-art light guide; and

FIG. 10 shows a sketch illustrating a projection parallel to the surfacenormal of the light guide, where at least two outcoupling elements areat least partially located under each sub pixel of said image generator.

The drawings are not to scale and illustrate principles only.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sketch illustrating the principle of light that is laterallycoupled into a light guide 3 by light sources 4 and outcoupled from thelower large surface of the light guide 3, with the outcoupling elements6 being provided on the said lower large surface. The outcoupled light,however, exits from the upper large surface of the light guide 3. In thehorizontal direction, the light is outcoupled from the upper largesurface of the light guide 3 in a large angle here. The locations of theoutcoupling elements 6 are suggested by the reference number 6, but theoutcoupling elements 6 proper are not shown, because they have to be ofmicroscopic size. Light, then, is coupled in laterally into light guide3 by the light sources 4, e.g., by LEDs. Due to total reflection, raysof the coupled-in light (represented by bold rays) are reflected by theouter wall and thrown back into the light guide 3 until they finally(probably after repeated hits) hit an outcoupling element 6 to undergothe desired outcoupling. This outcoupling is represented by the thinrays. For better understanding, the representation in FIG. 1 is highlyschematic; in reality, the light guide 3 has a vast number of ray paths.

FIG. 2 is a sketch illustrating the principle of light that is laterallycoupled into a light guide 3 by light sources 4 and outcoupled from theupper large surface of the light guide 3, with the outcoupling elements6 being provided on the said upper large surface. Here again, theoutcoupled light exits from the upper large surface of the light guide3. The comments on FIG. 1 apply accordingly.

What is technically different here is merely the location and possiblythe configuration of the outcoupling elements 6, which now occupy theupper side of the light guide 3, thus outcoupling the light directly upwithout the need to once again cross the light guide 3 in order to exitfrom it as shown in FIG. 1

FIG. 3 is a sketch illustrating the principle of light originating froma backlight 2 passing a light guide 3 through both of its largesurfaces, i.e. across its volume. Here, the outcoupling elements 6 playa part that is essentially negligible, because the light originates fromthe backlight 2, i.e., the light is not coupled into the light guide 3laterally through an edge from light sources 4, and therefore it is not,or scarcely, deflected back and forth by total reflection in the lightguide 3. Accordingly, the outcoupling elements 6 are not shown in thedrawing, since their effect in this constellation is negligible.

FIG. 4 is a sketch illustrating the principle of a screen 1 comprisingan illuminating apparatus with an image generator 5 with pixels whichare composed from R, G, B subpixels (hereinafter jointly representedunder the term of the viewed “screen 1”) in B1 mode for a free viewingmode.

The said screen 1 comprises

-   -   a backlight 2 of planar extension, radiating light into a        restricted angular range, the said restricted angular range here        meaning, e.g., an angular range of +/−30 degrees to the left and        right of the normal to the backlight 2 surface (in defining the        restricted angular range, small light quantities with maximum        brightnesses of less than 3% to 5% may possibly be left out of        account);    -   a transmissive image generator 5 arranged in front of the        backlight 2 as seen in the viewing direction;    -   a plate-shaped light guide 3 arranged between the image        generator 5 and the backlight 2, which is provided with        outcoupling elements 6 on at least one of its large surfaces        and/or within its volume, the light guide 3 being transparent to        at least 70% of the light emitted by the backlight 2,    -   light sources 4 arranged laterally on the edges of the light        guide 3, preferably on one or both of the two long edges of the        light guide 3, i.e., in a horizontal orientation (landscape        mode), on the top and bottom edges;    -   wherein the number of outcoupling elements 6—not shown in the        drawing—per surface area and their extension are selected such        that the light guide 3, on at least 50% of its surface, has an        average haze value of less than 7%, but preferably less that 2%        or, with special preference, less than 1%, measured according to        ASTM D1003, procedure A with a haze meter as reference, whereby        the light radiated by the backlight 2, at least in B2 mode in a        restricted angular range, is only negligibly, if at all,        scattered at angles outside the said angular range when it        passes the light guide 3;    -   wherein furthermore a distribution of the outcoupling elements 6        on at least one of the large surfaces and/or within the volume        of the light guide 3 is specified in such a way that light        irradiated by the light sources 4 into the light guide 3 and        coupled out of the light guide 3 by the outcoupling elements 6        satisfies the following conditions, without the need to use        additional optical layers such as diffusers, reflectors, etc.:        -   at least 50% of the quantity of light outcoupled at one of            the large surfaces in an angular range between −50° and +50°            relative to a peak luminance direction of the large surface            is radiated in an angular range between −20° and +20° with            respect to said peak luminance direction of the large            surface and relative to one or two specified directions that            are perpendicular to each other and to said peak luminance            direction, and/or at least 70% of the quantity of light            outcoupled at the large surface in an angular range between            −50° and +50° relative to said peak luminance direction of            the large surface is radiated in an angular range between            −30° and +30° with respect to said peak luminance direction            of the large surface and relative to one or two specified            directions that are perpendicular to each other and to said            peak luminance direction, and        -   at least 50%, preferably at least 60% or, with particular            preference, at least 80% of the quantity of light coupled            out of the light guide 3 is coupled out either in the            direction leading away from the backlight 2 or towards the            backlight 2, and        -   in a projection direction parallel to the surface normal of            the light guide 3, each pixel or sub pixel covers at least            two of the outcoupling elements at least partially, and        -   in said projection direction the fill factor of the            outcoupling elements 6 is 50% of the area of the large            surface of the light guide 3 or less,    -   wherein in mode B1 at least the light sources 4 are switched on.

The said specified directions are mutually perpendicular and lie in aplane on the light guide 3 or on the surface of the backlight 2. Inoperation, e.g. at a payment terminal or in a motor vehicle, the screenis fixed relative to its outer environment, so that the specifieddirections, irrespective of a portrait or landscape orientation of thescreen, can be assigned, e.g., the terms “vertical” and “horizontal”,which actually relate to the outer coordinate system of the environment.“Vertical” corresponds to the top-to-bottom, and “horizontal” to theleft-to-right direction on the screen.

It is possible here that in the B1 mode the backlight 2 is switched onor off. In FIG. 4, for example, the backlight 2 is off. If aparticularly bright mode is desired for the front view in the B1 mode,the backlight 2 may as well be on, which is not shown in the drawing.

FIG. 5 illustrates the principle of the invented screen 1 in the B2 modefor a restricted viewing mode; here, the backlight 2 is on, whereas thelight sources 4 are off.

The light radiated by the backlight 2 in a restricted angular rangeonly, represented by the bold arrows in FIG. 5, will then penetrate thelight guide 3 essentially without being scattered or deflected, asdescribed also in connection with FIG. 3, and subsequently illuminatethe image generator 5 in such a way that, due to its angularrestriction, the image generator 5 can only be seen from a restrictedangular range. The restriction of the angular range may apply to thehorizontal and/or the vertical direction.

Regarding the characteristic that the distribution of the outcouplingelements 6 on at least one of the large surfaces and/or within thevolume of the light guide 3 is given in such a way that 50% or more ofthe light quantity outcoupled from the light guide 3 is outcoupled inthe direction pointed away from the backlight 2, the following should benoted: In prior art, say, in utilizing nanoparticles such as, e.g.,titanium dioxide or polymerizate in the light guide volume, about halfof the light each is outcoupled from either of the large surfaces.This—and that is the point here—is not intended to be implemented by thepresent invention, because the light that is radiated at the backlight 2in prior art, can hardly be sent back to the image generator 5 and will,for the biggest part, get lost in the balance of utilization.

Compared to prior art, an enormous increase in efficiency with regard tolight gain is achieved by the fact that the distribution of theoutcoupling elements 6 on at least one of the large surfaces and/orwithin the volume of the light guide 3 is preset in such a way thatlight radiated by the light sources 4 into the light guide andoutcoupled from the light guide 3 by the outcoupling elements 6—withoutadditional optical layers such as diffusers, reflectors or brightnessenhancement films, which enhance the outcoupled light quantity only in anon-directional manner —satisfies the conditions that at least 50% ofthe light quantity outcoupled at one of the large surfaces in an angularrange between −50° and +50° relative to the normal to the large surfaceis radiated in an angular range between −20° and +20° related to one ortwo specified directions that are perpendicular both mutually andrelative to the surface normal, e.g., in the vertical and/or thehorizontal direction described above, and/or at least 70% of the lightquantity outcoupled at one of the large surfaces in an angular rangebetween −50° and +50° relative to the normal to the large surface isradiated in an angular range between −30° and +30° related to the one ortwo specified directions. This efficiency increase is due to the factthat the additional optical layers mentioned above cannot be used, as arule, since otherwise the B2 mode would be disabled due to scattering,refraction and possibly other optical effects.

Furthermore, according to the invention, in a projection directionparallel to the surface normal of the light guide 3 in the projectiondirection parallel to the surface normal of the light guide, eachpixel—or, if a pixel is composed of subpixels—subpixel covers at leasttwo of the outcoupling elements at least partially. This helps to reduceor even suppress optical artifacts that may occur due to the opticaloverlay of the structure of the outcoupling elements with the opticalstructure of the transmissive image generator 5 (e.g. an LCD panel),such as unwanted Moiré effects, color sparkling etc. Due to the opticalmixing of light passing through the at least two outcoupling elements inthe smallest image unit of the image generator 5, i.e. the pixels, or ifavailable, the color sub pixels (e.g. R, G, B), such artifacts aredecreased in visibility.

Additionally, the inventive feature that in a projection directionparallel to the surface normal of the light guide 3 the fill factor ofthe outcoupling elements 6 is 50% or less of the area of a large surfaceof the light guide 3, helps further to reduce optical artifacts and alsosupports the negligible disturbance of the propagation of the focusedlight in private operation mode B2.

The outcoupling elements 6 have maximum dimensions of 100 μm, preferablybetween 1 μm and 30 μm. Typically, the number of outcoupling elements 6per unit area varies across the surface of the light guide 3 in order toachieve the desired outcoupling properties such as, e.g., homogeneity.

The outcoupling elements 6 for light outcoupling from at least one ofthe large surfaces of the light guide 3 preferably comprise or consistof microlenses and/or microprisms and/or diffractive structures and/orthree-dimensional structural elements, their largest dimension having amaximum extension smaller than 35 μm, preferably smaller than 15 μm.

Thus, if the outcoupling elements 6 are provided on at least one of thelarge surfaces of the light guide 3, the said outcoupling elements areadvantageously formed from some plastic structured by means of a tool.This is possible in mass production, e.g., by applying a UV-curingmaterial (e.g., a lacquer or varnish, a monomer, etc.) onto a lightguide substrate, structuring the said material by means of a tool andcuring it by UV radiation, e.g., polymerization. Other eligiblemanufacturing processes are, e.g., injection molding, hot embossing andlithography.

Finally, the light guide 3 or its substrate may contain at least 40wt.-%, preferably at least 60 wt.-% polymethylmethacrylate, referred toits weight. Alternatively, the material contained can be polycarbonate(PC), e.g.

In another favorable embodiment of the invention, in mode B1, as afunction of specified limiting angles σ, γ, the outcoupled light exitingfrom the light guide 3 at an angle β will, at every point of the lightguide surface in angular ranges satisfying the conditions of 80°>β>γand/or −80°<β<−σ, with 10°<γ<80° and 10°<σ<80° and preferably withγ=σ=40°, measured normal to the light guide 3 surface and in at leastone of the two specified directions, e.g., in a vertical orientationrelative to the surface of the light guide 3 (orientation here means thealignment of the light guide 3 and, thus, inherently also of the imagegenerator 5, especially a portrait or landscape orientation), havemaximally 80%, preferably maximally 60% or, with particular preference,maximally 50% of the luminous intensity of the light exiting from such apoint of the surface of the light guide 3 normal to that surface, i.e.along the surface normal. Thereby, one achieves, e.g., a reduction ofdisturbing reflexes in the windshield, especially in night-time driving,if the invented screen is installed in a motor vehicle. The limitingangles σ, γ are specified and fixed in accordance with the desiredapplication of the illuminating apparatus, e.g. in a motor vehicle or ina laptop.

Concerning this, FIG. 6 is a schematic representation of defining thevertical direction of the angle β to be measured, wherein the lightguide 3 (as also the image generator 5 not shown) is arranged in thelandscape mode, i.e., the long edges are at the top and the bottom. Thedash-dot line represents a direction normal to the surface of the lightguide 3, relative to which, in a vertical orientation or plane, markedhere by the dual-pointed arrow lettered “V”, the angle β is measured.

FIG. 7, then, is an example of a graph of the relative brightness of thelight outcoupled from the light guide 3, measured in B1 mode in verticaldirection. The abscissa represents the angle β, and the ordinaterepresents a relative luminance measured in vertical direction at therespective angle β. It is clearly visible that, as described above forthe B1 mode, the light emitted by the light sources 4 and exiting fromthe light guide 3 at the selected measuring point on its surface atangles β>40 degrees and/or β<−40 degrees, has maximally 50% (here, evenless than about 25%) of the light intensity of the light exiting fromthe selected point of the surface of the light guide 3 normal to itssurface, the normal of which equals to said above predefined direction.

FIG. 9 shows the brightness distribution along a specifieddirection—here, the vertical direction accordingly positioned in ahigher-rank coordinate system—across an angular range of the exit angleβ for a light guide of prior art, this distribution resulting withoutany additional layers such as diffuser layers, brightness enhancementfilms or reflector layers otherwise common in a backlight. Whereas inprior art these additional layers arrange for adapting the lightdistribution parameters to given values, they cannot be used in theilluminating apparatuses and screens described above and below, becauseoperation in B2 mode would not be possible then, because these layers,due to their non-directional, statistical radiation, eliminate anyprivacy effect.

Another practical embodiment of the invented screen 1, as shown in FIG.8, includes another light guide 5 a that is arranged (seen in theviewing direction) in front of the image generator 5 and provided withmeans for light outcoupling, and into which light can be fed laterallyby light sources 4 a. The outcoupling means used here are, e.g., thosedescribed above for the light guide 3. With this embodiment, anyunintentional residual light in Mode B2 in angular ranges actuallyprotected against viewing can be interfered with or glared (see thethin, oblique arrows in FIG. 8), so that no contrast is visible and noimage can be perceived from the disallowed angles.

The light sources 4 a eligible are adapted to radiate colored or whitelight, and they can radiate light of a color that is not present in theimage displayed by the transmissive image generator 5.

Further, this light can optionally be radiated at various brightnesslevels. Moreover, it is possible for the chromatic properties, e.g. thecolor and/or brightness, of the light emitted by the light sources 4 ato be modulated in time. In addition, the light sources may comprisedifferent light sources 4 a, such as, e.g., RGB LEDs in LED rowsradiating light of different colors and/or different brightnesses eithersimultaneously or at different times, and/or staggered.

The light guide 5 a, especially if it uses, in a suitable way,outcoupling elements 6 similar to those described for the light guide 3,may just as well outcouple light in one selected direction only, e.g.,to the left or the right, but almost not, or to a negligible extentonly, normal to its surface. This has the advantage that the imagecontrast is, for the person directly in front of the screen in mode B2,is not, or almost not, reduced, whereas privacy protection sideways ismarkedly improved as described above.

Furthermore, the said restricted angular range may be configured eithersymmetrically or asymmetrically relative to the surface normal of theimage generator 5. The latter is helpful, in particular, in motorvehicle applications, e.g., if the screen 1, constituting a so-calledcenter information display, is arranged on the dashboard about halfwaybetween the driver and the front-seat passenger. In this case, therestricted angular viewing range that is exclusively free for thepassenger must be configured asymmetrically, i.e. directed towards thepassenger.

The backlight 2 may, e.g., consist of

-   -   a planar emitter, preferably a light guide with light sources        such as LEDs arranged at its edges or on the rear surface, and    -   two light collimators crossed at right angles and integrated in        the planar emitter and/or arranged in front of it, such as,        e.g., “Optical Lighting Film”™ made by 3M™ or prism arrays, and        at least one privacy filter, also made, e.g., by 3M™, arranged        in front of it.        Using a so-called directed backlight as a backlight illuminator        2 is possible as well.

Finally, FIG. 10 shows a sketch illustrating the projection parallel tothe surface normal of the light guide 3. Seen along a projectiondirection parallel to the surface normal which in FIG. 10 coincides withthe normal of the paper plane, under each subpixel (here indicated as R,G, B) of the image generator 5 are located at least two outcouplingelements 6 at least partially with their surfaces.

It is particularly advantageous to use the invented screen 1 in avehicle for selectively displaying image contents for the front-seatpassenger only in B2 mode, or simultaneously for the driver and thefront-seat passenger in B1 mode. The former applies to the case that,e.g., the front-seat passenger is watching entertainment contents thatcould distract the driver's attention.

In all embodiments described above, the said light sources 4 or 4 a maybe LEDs or LED rows or laser diodes. Other versions are feasible as welland are within the scope of the invention.

The above-described illuminating apparatus according to the inventionand the screen that can be implemented therewith solve the problemsaddressed and permit to be readily translated into practical solutionsproviding privacy-proof presentation of information by means of anoptionally restricted viewing angle and, in a separate operating mode,free viewing without any restriction of the viewing angle. The inventioncan be put into practice by simple means and at affordable cost. In bothoperating modes, the native resolution of the image display deviceemployed can be utilized. Moreover, the solution causes but little lightloss.

The invention described above can be used to advantage whereverconfidential data are displayed and/or entered, such as in entering PINsor passwords, data display at ATMs or payment terminals, or readinge-mails on mobile devices. As also described above, the invention canalso be used in cars.

1. A screen, comprising a transmissive image generator with pixels andan illuminating apparatus, wherein the screen can be used in at leasttwo operating modes, B1 for a free viewing mode and B2 for a restrictedviewing mode, the illuminating apparatus comprising: a backlight ofplanar extension that radiates light in a restricted angular range, aplate-shaped light guide that is located in front of the backlight asseen in a viewing direction and is provided with outcoupling elements onat least one of its large surfaces and/or within its volume, wherein thesaid light guide is transparent to at least 70% of the light emitted bythe backlight, light sources arranged laterally at edges of the lightguide, wherein a number of outcoupling elements per surface area andtheir extension are selected such that the light guide, on at least 50%of its surface, has an average haze value of less than 7%, measuredaccording to ASTM D1003, whereby the light radiated by the backlight atleast in B2 mode in a restricted angular range is only negligibly, if atall, scattered at angles outside the said angular range when it passesthe light guide, wherein in mode B2 the backlight is switched on and thelight sources are switched off, whereas in mode B1 at least the lightsources are switched on, and a distribution of the outcoupling elementson at least one of the large surfaces and/or within the volume of thelight guide is specified in such a way that light irradiated by thelight sources into the light guide and coupled out of the light guide bythe outcoupling elements satisfies the following conditions: at least50% of the quantity of light outcoupled at one of the large surfaces inan angular range between −50° and +50° relative to a peak luminancedirection of the large surface is radiated in an angular range between−20° and +20° with respect to said peak luminance direction of the largesurface and relative to one or two specified directions that areperpendicular to each other and to said peak luminance direction, and/orat least 70% of the quantity of light outcoupled at the large surface inan angular range between −50° and +50° relative to said peak luminancedirection of the large surface is radiated in an angular range between−30° and +30° with respect to said peak luminance direction of the largesurface and relative to one or two specified directions that areperpendicular to each other and to said peak luminance direction, atleast 50% of the quantity of light coupled out of the light guide iscoupled out either in a direction leading away from the backlight ortowards the backlight, and in a projection direction parallel to asurface normal of the light guide, each pixel covers at least two of theoutcoupling elements at least partially and in said projectiondirection, a fill factor of the outcoupling elements is 50% of an areaof the large surface of the light guide or less.
 2. The screen asclaimed in claim 1, wherein the light guide consists of a transparentpolymer, a thermopolymer, plastic or thermoelastic material or of glass.3. The screen as claimed in claim 1, wherein the distribution of theoutcoupling elements on at least one of the large surfaces and/or withinthe volume of the light guide is specified in such a way that the lightcoupled out on at least 70% of the surface of the light guide attains ahomogeneity of the luminance of at least 70%.
 4. The screen as claimedin claim 1, wherein the outcoupling elements have maximum dimensions of100 μm.
 5. The screen as claimed in claim 4, wherein the outcouplingelements have maximum dimensions of between 1 μm and 30 μm.
 6. Thescreen as claimed in claim 1, wherein the outcoupling elements comprisemicrolenses and/or microprisms and/or diffractive structures and/orthree-dimensional structural elements and/or scattering elements.
 7. Thescreen as claimed in claim 1, wherein the outcoupling elements arefashioned within the volume of the light guide, and the outcouplingelements are fashioned as cavities that have the outer shape ofmicrolenses, microprisms or diffractive structures.
 8. The screen asclaimed in claim 7, wherein the cavities are filled with some gaseous,liquid or solid material, the material having a refractive indexdeviating from that of a refractive index of a material used for thelight guide, or wherein the cavities form vacuums.
 9. The screen asclaimed in claim 8, wherein the refractive index of the material in thecavities is lower than the refractive index of the material used for thelight guide.
 10. The screen as claimed in claim 8, wherein the cavitiesare filled with some gaseous, liquid or solid material, the materialhaving a haze value deviating from a haze value of a material used forthe light guide.
 11. The screen as claimed in claim 10, wherein the hazevalue of the material in the cavities is higher than the haze value ofthe material used for the light guide.
 12. The screen as claimed inclaim 9, wherein the light guide is made of two similar substrate layersjoined by their boundary surfaces, and the cavities are created asmaterial recesses in at least one of the boundary surfaces, the cavitieshaving an outer shape of microlenses, microprisms, three-dimensionalstructural elements or diffractive structures.
 13. The screen as claimedin claim 1, wherein the outcoupling elements are provided on at leastone of the large surfaces or boundary surfaces of the light guide, andare made of some polymer, plastic or glass structured by means of atool, such as by a mechanical, lithographic, printing, materialdepositing, material abrading, material converting or materialdissolving process.
 14. The screen as claimed in claim 1, wherein thebacklight comprises: a planar emitter comprising a light guide withlight sources arranged laterally or on the rear surface, and at leastone light collimator integrated in the planar emitter and/or arranged infront of the planar emitter.
 15. The screen as claimed claim 1, whereinin mode B1, as a function of specified limiting angles σ, γ, theoutcoupled light exiting from the light guide at an angle β will, atevery point of the light guide surface in angular ranges satisfying theconditions of 80°>β>γ and/or −80°<β<−σ, with 10°<γ<80° and 10°<σ<80measured against a predefined direction passing through a midpoint ofthe surface of the light guide and in at least one of the two specifieddirections, has maximally 80% of the light intensity of the lightexiting from such a point of the light guide surface along saidpredefined direction.
 16. The screen as claimed in claim 15, whereinγ=σ=40°.
 17. The screen as claimed in claim 15, wherein the outcoupledlight has maximally 50% of the light intensity of the light exiting fromthe point of the light guide surface along said predefined direction.18. The screen as claimed in claim 1, wherein the transmissive imagegenerator is arranged, in the viewing direction, in front of the lightguide.
 19. The screen as claimed in claim 1, further comprising a secondlight guide that includes outcoupling elements and is arranged, in theviewing direction, in front of the image generator, and is configured toreceive light laterally from light sources.
 20. The screen as claimed inclaim 1, wherein the image generator is provided with pixels that areassemblies of subpixels, and wherein each dimension of the outcouplingelements in height, depth and width is smaller than a minimum of widthand height of the subpixels of the image generator.
 21. The screen asclaimed in claim 1, wherein the image generator is provided with pixelsthat are assemblies of subpixels, and wherein in the projectiondirection parallel to a surface normal of the light guide, each subpixelcovers at least two of the outcoupling elements at least partially. 22.A use of a screen as claimed in claim 1 in a vehicle for selectivelydisplaying image contents either for a front-seat passenger only inoperating mode B2 or simultaneously for a driver and the front-seatpassenger in operating mode B1.